Methods of achieving two-dimensional (2D) active dimming for televisions and comparable large area displays made up of direct lit light-emitting diodes (LEDs) behind a liquid crystal display (LCD) are known. Miniaturization of active dimming technology to mobile size displays, such as for example smartphones and tablets, involves replacing a lightguide of low thickness with an LED array. In addition, less LEDs are needed than viewing zones required for high quality high dynamic range (HDR) displays. Effective active dimming for mobile displays enhances power management and in particular can extend battery life.
Various attempts have been made to minimize display size in a manner that can be optimized for mobile devices that utilize active dimming. Such attempts have encountered issues, including visibility of zones associated with visible edge patterns of the electrode structures. US 2012/0007896 (Kwon et al., published Jan. 12, 2012) describes the issues with zone visibility in active dimming systems and proposes a method to reduce zone visibility using image processing. As to additional issues associated with minimizing display size, JP 2012129105 (Murata et al., published Jul. 5, 2012) uses optical elements on each direct lit LED to reduce thickness. U.S. Pat. No. 8,199,280 (Kim et al., issued Nov. 26, 2009) attempts to create a thinner direct lit 2D backlight by using individual lightguide elements. U.S. Pat. No. 5,686,979 (Weber et al., issued Nov. 11, 1997) and U.S. Pat. No. 8,248,555 (Senoue et al., issued Aug. 21, 2012) attempt to create a switchable aperture using a second LC panel over the standard backlight.
Effective active dimming with miniaturization generally is not achieved for such devices. Miniaturizing a direct lit, large-display backlight to a mobile size suffers from two principal problems. First, the thickness is substantially reduced which means uniformity is difficult to achieve with a large number of LEDs, and second, the number of LEDs that can be used is less than the number of zones needed. Using lightguides or other optical elements on the LEDs of a direct lit backlight does reduce thickness, but not generally to an extent that can make the backlight mobile. In addition, the use of lightguides or other optical elements does not solve the zone number issue.
Using a second LC aperture (usually a passive matrix LC panel) on a normal backlight produces good quality zones at a low thickness suitable for mobile devices. In such configurations a passive matrix LC and polarizer (or just a reflective polarizer sheet) is placed between the LCD and a normal backlight to create a 2D array of high contrast zones, which can be controlled easily. The number and distribution of zones is then independent of the light source. Conventional configurations, however, encounter a zone visibility problem as referenced above. As the passive matrix LC is immediately behind the main display and separated by a significant thickness, zone edges of the passive matrix LC elements are well defined and sharp, and such edges may be seen in different places on the main display because of parallax between the eyes. This means that the image processing algorithm edge cannot perfectly line up with the LC edge. This results in undesirably generating a rectangular edge pattern that can be seen in the image. The generation of such rectangular edge pattern cannot be corrected well in software; nor is it practical to use a diffuser between the passive matrix LC and the main panel. A diffuser would cause the brightness from the backlight to be significantly reduced.